Research in this laboratory has dealt principally with (1) definition of the shortest amino acid sequence of the guinea pig myelin basic protein capable of inducing experimental allergic encephalomyelitis in Lewis rats, and (2) analysis of chemical modifications introduced into the basic protein when the polypeptide chain is cleaved at the tryptophan residue by BNPS-skatole. In the first study we have found that the encephalitogenic activity is associated with a sequence of 12 residues located approximately in the middle of the 167 residue protein. In the second study we have found that cleavage of the basic protein with BNPS-skatole results in partial bromination of tyrosine residues. Methods of analysis of brominated tyrosine involving ultraviolet absorption spectra, ion-exchange chromatography, and dansylation have been developed. Chemical cleavage at the tryptophan residue is accompanied by the formation of multiple tryptophan oxidation products which introduce a chromatographic and electrophoretic heterogeneity into the cleaved polypeptide chain. Bromination of tyrosine and the creation of multiple tryptophan oxidation products may complicate isolation and analysis of the peptides and alter their immunogenicity.